Aircraft: EMB ERJ 145 ER/LR
Phase of flight: Parked
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury; Ground Event / Encounter Loss Of Aircraft Control

We were operating a morning first flight of day. The aircraft was put on a hardstand with no tug where with guidance from the ramp crew we would be able to start both engines and be directed to the movement area of the ramp. After completing the Engine Start Check; I looked over to the Ramper and with their permission; I released the parking brake. I however; did not reengage the parking brake. I then directed the First Officer to start both engines. The number 2 engine was started first. Following the start of the number 1 engine; the First Officer stepped on the brakes and simultaneously engaged the parking brake. This caused the airplane to stop abruptly. I asked the First Officer why he did what he did; and he said that we were moving. I then said that it was my mistake for not having the parking brake engaged; but we were moving slowly and we were far from any equipment and people. I then asked the First Officer two questions Why did he not say to me that the aircraft was moving and I would have applied the brakes and why did he apply both the brakes and the parking brake at the same time ? He said that he was not sure the brakes alone would have stopped the airplane. We then proceeded with the rest of the normal checklists and taxied to the runway and departed. In flight; the Flight Attendant said she was injured from the whiplash and was bruised. She said she may see a doctor upon reaching our destination. At the end of the flight I exchanged my contact information with the Flight Attendant to keep me informed of her physical condition. The whole sequence of events started with me not engaging the parking brake after releasing it for the out time. The threat of not recognizing that we do not have rampers to tell us to reset the brake after push was not addressed during our threat discussion and mitigation. My error was not seeing the airplane moving after the number one engine started and stopping the airplane before the First Officer. The undesired state was the fact that the aircraft moved when it should not have causing potential harm to the ramp crew and outside equipment. The other error I would say was CRM. The First Officer acted in the interest of safety and was the first to recognize the movement of the aircraft. He however over reacted. Based on the fact the airplane was moving slowly and we were far from rampers and equipment; I think he should have told me that we were moving and I would have stopped the airplane. If I did not react he then should have stepped on the brakes. This was the error on CRM our part. After talking to him I found out that he just finished IOE recently and this was his first trip after completing IOE. I think lack of experience in actual line operations caused him to act the way he did. I also should have checked with the Flight Attendant to see how she was doing after the airplane came to a stop. What I learned was to be extra vigilant when you are taxiing out with no towbar from the gate. If we were close to people or equipment; this could have been bad. Addressing ramp operations with no tug driver telling you to 'set the brake' is a threat that we should have discussed and talked about. I also need to be more aware of aircraft (unintentional) movement in all situations and react appropriately. ERJ145 Captain reported the parked aircraft began rolling on the ramp and the First Officer put the brakes on and the parking brake causing an abrupt stop injuring a Flight Attendant.